1. Field of the Invention
The present invention relates to a polyurethane film of high optical quality with energy-absorbing properties, scratch resistance and abrasion resistance, and antifogging properties, that can be used as an external film in safety windows, especially windshields.
2. Description of the Background
European Patent No. 0 133 090 discloses a polyurethane film formed in a continuous process by reactive casting of a reaction mixture of the components of a polyurethane on a flat, horizontal support from which it can be detached. Reactive casting in this technical field means casting to form a film from a liquid mixture of components in the state of monomers or prepolymers, followed by polymerization of this mixture by heat. The reaction mixture described in the patent includes an active hydrogen component, specifically a polyol component, and an isocyanate component containing at least one aliphatic or cycloaliphatic diisocyanate or a diisocyanate prepolymer, with this component having a viscosity below approximately 5000 centipoises at +40.degree. C. The polyol component contains at least one long difunctional polyol with a molecular weight between 500 and 4000, at least one short diol as a chain extender, and in some cases a small proportion of at least one polyol having a functionality greater than 2, especially aliphatic triols. The proportions of the long polyol, the short diol, and possibly the polyol with functionality greater than 2, are generally chosen so that for one hydroxyl equivalent, the long polyol represents about 0.30 to 0.45 equivalent, the short diol about 0.2 to 0.7 equivalent, and the polyol having a functionality greater than 2, about 0 to 0.35 equivalent. Under these conditions, the film produced has the following mechanical characteristics, measured according to AFNOR/NFT standards 46 002, 51 034, and 54 108.
(i) a yield stress .sigma..sub.y at -20.degree. C. less than or equal to 3 daN/m-.sup.2
(ii) a breaking stress .sigma..sub.R at +40.degree. C. greater than or equal to 2 daN/mm.sup.2,
(iii) an ultimate elongation .epsilon..sub.R at +20.degree. C. between 250 and 500%,
(iv) a tear propagation strength Ra at +20.degree. C. greater than or equal to 9 daN/mm of thickness.
The film product of the patent also has energy-absorbing properties, as mentioned above, and scratch and abrasion resistance that make it suitable for use as an external film under some conditions. Thus, it has scratch resistance above 20 grams and abrasion resistance with blurring differential below 4%, with these values being measured by the tests described in the cited publication.
However, under severe conditions of use, for example when the film is used externally in automobile windows, where it is subjected to attack from a variety of sources and in particular attack from certain cleaning products containing strong organic solvents, the film is not entirely satisfactory, since its resistance to these products is inadequate. Furthermore, the polyurethane films mentioned above, when combined with a glass or plastic support and placed under conditions of high humidity, may become covered with fog rapidly, like glass windows, which hinders vision through the window. A need therefore continues to exist for a polyurethane film of good optical qualities and stability properties.